1. Field of the Invention
This invention relates to X-ray apparatus and more particularly to an electrical system for automatically preventing X-ray exposures under overload conditions.
2. Description of the Prior Art
Modern high power X-ray tubes normally include a tungsten filament encased in a cathode cup which is mounted a short distance away from a rotating tungsten anode. The anode, in turn, is connected to a motor armature and bearing assembly with the entire structure mounted within a glass envelope of the X-ray tube. The tube is placed such that the motor armature and that portion of the glass envelope surrounding it are within the motor stator winding. When the winding is energized, the anode rotates so that during the X-ray exposure, new areas of the anode are brought within the electron beam cross section. The thermal capacity of the tube, i.e., the maximum X-ray exposure possible without damage to the anode, is determined by the energy lever per exposure which is a function of the peak power kW expressed in terms of voltage (kV) and current (mA) and the exposure time in seconds (sec.), the area on the anode subtended by the electron beam as well as the shape and finally the speed of rotation of the anode.
In an effort to obtain the maximum output per exposure of a particular X-ray tube, the manufacturer attempts to rate the respective tube at the maximum possible value per exposure such that the anode is brought almost to the point of melting during each X-ray exposure. In order to do this, the manufacturer publishes curves called "rating charts" which describe the maximum exposure for a particular X-ray tube under the various conditions of operation.
Prior art protective circuits provide a control circuit which is responsive to analog voltages that are functions of the desired power level and the exposure time duration, respectively, and being then operative to compare the manual setting made by the operator with predetermined known maximum safe values allowed by the tube rating chart curve for the particular X-ray tube employed. These analog voltages are generated by means of suitable power supply voltages feeding two voltage divider networks, one of which corresponds to X-ray tube power, while the other corresponds to X-ray exposure time. If the X-ray generator operates for example with three X-ray tubes, and if each X-ray tube has two different focal spot sizes and further if the X-ray tube anodes are permitted to rotate at two different speeds (standard speed and ultra speed) separate voltage divider switch decks for exposure time (numbering 12 in total) are normally required and which are ganged on the X-ray exposure time switch shaft in order to simulate each rating chart curve for the particular mode of operation desired.
One approach to the problem is suggested in the teachings of U.S. Pat. No. 3,838,285, entitled "X-ray Tube Anode Protective Circuit," M.P. Sieband, et al. and assigned to the assignee of the present invention. The protective circuit there utilizes a single empirically derived anode rating chart curve which is representative of a generalized or standard X-ray tube rating chart curve. This empirical curve is generated by a series connected string of resistors connected to the exposure time select switch. This curve is then tilted and/or offset for selected operating modes in order to substantially conform to the actual rating curve for the respective various focal spot sizes and anode speeds of the particular X-ray tube in use. While this is acceptable for certain applications, it nevertheless becomes undesirable where a more exact conformance to the actual rating chart curve is desired.